Jose Mourinho suffered his first defeat since his return to the Premier League at Goodison Park as Everton hung on to a late-first-half goal for a 1-0 win on Saturday. Steven Naismith's header moments before half-time proved decisive as Chelsea wasted numerous chances and struggled to break down the Toffees midfield.
The Blues may lament an unawarded penalty when Oscar was caught in a defensive sandwich on the edge of the box, but the fact remains that Everton were able to execute their game plan more effectively.
Mourinho will have taken plenty of notes to work on, but here are six things we learned from this disappointing defeat.
Statistics taken from Four Four Two StasZone.
Mourinho told the press in his pre-match conference that he would use two strikers when he needed the win, and he did just that against Everton. Well, he brought Fernando Torres on in place of Ashley Cole with 20 minutes to go, and that's theoretically the same thing.
Although it didn't pay off, in large part due to some excellent Evertonian defending, the willingness to change the system will serve the side well throughout the season.
All of the talk about Mata not fitting into Mourinho's plans was just that: talk. Nonsense talk at that.
Mata started at Goodison Park and had one shot off target, completing 88 percent of his passes. He was replaced by Oscar after 57 minutes, indicating that he is still lacking match fitness. With a long season ahead, and the way that the Blues completely relied on him last year in mind, expect the little magician to ease his way back into the side in the coming weeks.
Ashley Cole was at fault for Steven Naismith's goal on the stroke of halftime. Nikica Jelavic was unmarked at the far post as Cole found himself stranded, leaving him free to head the ball across goal for the birthday boy to nod home.
Cole's reading of the game today was nowhere near his usual exceptional standards, and his error ultimately led to Chelsea's defeat. With Ryan Bertrand not yet at the level needed to take over left-back duties full time, Mourinho should start scouting for an interim replacement in the January window in case an extension to the 32-year-old's contract does not materialise.
Much was made of Everton's decision to allow Marouane Fellaini to follow David Moyes to Manchester United in the final few hours of the transfer window with little time to find a replacement. However, Gareth Barry proved to be more than adequate, putting in a "man of the match" performance.
Barry appeared from nowhere to block what should have been Samuel Eto'o's first goal as a Chelsea player in the first half and managed to stay out of Howard Webb's book despite committing five fouls. He and the rest of his teammates were constantly hassling and hurrying Chelsea into making the wrong decision, preventing the midfield from playing their pretty passing game by closing them down.
Everton won 18 of 26 take-ons attempted compared to Chelsea's nine of 23. They effectively smothered the Blues counter-attack in a confident display, despite the absence of the big-haired Belgian.
In the closing stages of the game, Samuel Eto'o summed up his afternoon when, after a fine display of his dribbling ability he was unable to beat James McCarthy for pace to get his shot away.
His finishing lacked the usual finesse and he was missing a yard of pace, but his six attempts show that he still has the hunger to score. Once he has adjusted to the rhythm of the Premier League he will doubtless return to his usual prolific form, but this was by no means the most impressive debut of his career.
While Sir Alex Ferguson was famed for his hair dryer treatment, Jose Mourinho has kept a certain mystery about his halftime and post match team talks.
However, the second-half fight backs that characterised his first spell in charge at Chelsea suggested a stern talking to forms part of his management style. His post match comments today clearly showed that he was disappointed in his side's efficiency today.
The Blues had 22 shots on goal, six of which were on target, and you can bet your last fiver that the squad will be working on that in training ahead of their next match on Wednesday. The sloppy passes will be analysed, the poor positioning will be tightened up, and Mourinho will motivate them to come back fighting.